The nakedness of your daughter-in-law you shall not reveal: To not have intercourse with the wife of the son, which is his wife from the betrothal — and all the more so from the marriage — whether during his lifetime and even after he divorced her, or whether after his death. In any manner [above], she is called the wife of his son and she is forbidden to him, as it is stated (Leviticus 18:15), “The nakedness of your daughter-in-law you shall not reveal.” And from that which appears, the liability is even with a son that is a mamzer — he is his son regardless. But the son from a maidservant or from a gentile is not called “his son” for anything.

We have written the root of the prohibition (Sefer HaChinukh 190).

From the laws of the commandment is, for example, that which they, may their memory be blessed, forbade (Yevamot 21b) — to make a fence for this — the daughter-in-law of his son and likewise the daughter-in-law of the son of his son, to the end of the world. And they also forbade only the daughter-in-law of his daughter. And the reason is similar to that which we said above — since the prohibition came in the verse about his daughter-in-law, they decreed about all daughters-in-law that come on account of the son; and they decreed with [the daughter-in-law of] the daughter only, and that is enough for us with that. And these are called secondary [prohibitions].

And this prohibition is practiced in every place and at all times. And one who transgresses it and laid with his daughter-in-law volitionally — and there are witnesses and a warning — is [punished] with stoning; and if there are no witnesses, with excision; inadvertently, he brings a fixed sin-offering. And if he laid with the daughter-in-law of his son or the son of his son, to the end of the world — or the daughter-in-law of his daughter — we strike him with lashes of rebellion.